Evolution of the mechanical damage of concrete SFRC under flexion load through the analysis of the speed of the ultrasonic pulse

Abstract

Introduction: In this article, an analysis of the evolution of the mechanical damage of steel fiber reinforced concrete (SFRC) and propagated ultrasonic pulse velocity (UPV) is performed; in order to correlate the UPV and mechanical damage in concrete elements with different dosages of steel fibers. For which 45 concrete samples were manufactured, with dosages of 0 kg/m³; 25 kg/m³ and 70 kg/m³, and average compressive strengths [1] of 57 MPa, 53 MPa and 44 MPa and flexural strength [2] of 5.49 MPa, 7.20 MPa and 9.60 MPa, is to clarifying that in the tested elements load, deflection and ultrasonic pulse velocity were measured in simultaneously; obtaining parameters that allowed to generate predictive equations of the damage, on a scale of 0 to 1, taking into account that a mechanical damage is advanced when an element has a high cracking and therefore its mechanical resistance capacity will be reduced (when the value is close or equal to 1) [3]; Additionally, with the completion of the project, it is demonstrated that the inclusion of steel fibers causes ductile behavior in the concrete with an increase in mechanical strength to flexion and fatigue. Concluding that it is possible to predict the mechanical damage of concrete by applying indirect measurement techniques, which will help the pathological evaluation of elements, as well as demonstrating the usefulness of using steel fibers in concrete to improve its mechanical properties.

Objective: Estimate the evolution of the mechanical damage of the SFRC-type concrete, by measuring the ultrasonic pulse velocity, correlating with the flexural strength, obtaining damage prediction equations and graphs of mechanical behavior.

Method: For the realization of the project an experimental model is applied; starting with a primary search for information on fiber-reinforced concretes and indirect inspection methods, then the concrete mixtures that are to be made were defined and the manufacturing of the test elements was carried out; then the experimental tests are carried out and the results are analyzed; finally, to realize conclusion of the research.

Results: Predictive and graphical equations of mechanical behavior are obtained, which will allow an insight of the mechanical damage of the concrete to be inspected in the first instance, which would allow establishing the physical state in the that an element made of this material is found.

Conclusions: As mechanical damage occurs in a concrete element, these will increase its cracking and therefore its density will decrease; This feature can be measured with the pulse rate propagated; consequently the prediction of the integrality of the physical state in which an element is found will be predictable, allowing to know if a particular structure is at risk of collapse or close to it; The foregoing becomes more important in structures subjected to flexion (property in which the concrete offers low resistance, with respect to compressive strength), such as a bridge, a vehicular track or beams of a building. With the equations set forth, it is shown that in a mechanical damage of less than 0.5, the structure cracks, but will not collapse, but if the damage is greater than 0.5 the structure is so fissured that it will not be able to withstand the load maximum design and therefore can collapse. In the same way, the project shows that steel fibers increase the flexural strength and toughness of the concrete by at least 30%, which implies its usefulness for the construction of elements described above.

This research is a first statement and will allow expanding the possibilities of indirect inspection methodologies in the field of structural study in Civil Engineering, as well as the use of steel fibers as additions, which improve the mechanical properties of concrete.